Like all complex machinery, aircraft design is highly iterative. Blueprints will be drawn and redrawn dozens of times over the course of an aircraft’s development, and this holds true for an aircraft’s pylons as well. The pylon, the structure that attaches the aircraft’s engines to its wings, is a critically important part of the fuselage, and so goes through multiple iterations.
The process begins with the aerodynamicists designing the master surfaces of the aircraft’s master surfaces, i.e. the general shape of the aircraft. Structure designers must then work around the space requirements imposed by this design, but compromises between the two design teams will often be needed as the aircraft is designed.
There are multiple factors that go into designing the pylons. The first and most major factor is the operating envelope, the aircraft’s performance characteristics such as top speed and service ceiling that dictate the aerodynamic loads on the pylon. Performance numbers such as thrust and weight can be obtained from the engine, and design load factors such as crash and emergency features must be accounted for as well.
The exact design of the pylon will depend on these figures. Pylon design uses similar structures of cable OEM (original equipment manufacturer) based on that company’s experience, expected mathematical models, and flight test data. Most pylons are based on proprietary structural arrangements used in previous designs. Boeing, for instance, has a proprietary design they do not share with the general public. From this basic design, the unique loads and configuration of the assemblies going into the pylon will be calculated by a stress engineering group into sizing for the geometry of the assemblies. This first pass at the assembly design will be done by the pylon group, then used as a basis of stress for sizing, going through an iterative process that will be repeated until an acceptable design solution is found.
Ground tests and test flights will be the ultimate proof of design. Afterwards, the manufacturer will finish with a weight reduction process to reduce part geometries of excess material once the actual flight loads have been verified.